POINT-SYMMETRIC MACH-ZEHNDER-INTERFEROMETER DEVICE

Abstract

The present invention provides a Point-Symmetric Mach-Zehnder-Interferometer (PSMZI) device, comprising three consecutive path delay sections (PDSs) provided as two outer PDS and one center PDS, each PDS including an upper waveguide arm and a lower waveguide arm. The PSMZI device also includes four asymmetric couplers (ACs) each AC including an upper waveguide portion and a lower waveguide portion. One AC is arranged directly on each side of each PDS, the upper and lower waveguide portions being respectively coupled to the upper and lower waveguide arms. Further, the AC on the one side of the PDS is point-symmetric to the AC on the other side of the PDS, and the two ACs and the one outer PDS arranged on the one side of the center PDS are together point-symmetric to the two ACs and the one outer PDS arranged on the other side of the center PDS.

Claims

1. A point-symmetric Mach-Zehnder-Interferometer (PSMZI) device, comprising: three consecutive path delay sections (PDSs) provided as two outer PDSs and one center PDS, each PDS comprising an upper waveguide arm and a lower waveguide arm; four asymmetric couplers (ACs) each comprising an upper waveguide portion and a lower waveguide portion; wherein one AC is arranged directly on each side of each PDS, the upper and lower waveguide portions being respectively coupled to the upper and lower waveguide arms; wherein the AC on the one side of the PDS is point-symmetric to the AC on the other side of the PDS, and wherein the two ACs and the one outer PDS arranged on the one side of the center PDS are together point-symmetric to the two ACs and the one outer PDS arranged on the other side of the center PDS.

2. A PSMZI device according to claim 1, wherein the center PDS provides a path difference of zero.

3. A PSMZI device according to claim 1, wherein a path difference provided by one outer PDS is the same, but is provided in the other waveguide arm, than a path difference provided by the other outer PDS.

4. A PSMZI device according to claim 1, wherein a total path length of all upper waveguide arms is the same as a total path length of all lower waveguide arms.

5. A PSMZI device according to claim 1, wherein the four ACs and the two outer PDS are designed such that a phase difference, which is caused by the two ACs and one outer PDS arranged on the one side of the center PDS, is compensated by a phase difference, which is caused by the two ACs and one outer PDS arranged on the other side of the center PDS.

6. A PSMZI device according to claim 1, wherein the four ACs are line-symmetric series-tapered (LSST) type.

7. A PSMZI device according to claim 1, wherein the waveguide arms are made of a material having a refractive index in a range of 1.4-4.5.

8. A PSMZI device according to claim 1, wherein the waveguide arms are made of SiN and are embedded into a cladding made of SiO.sub.2.

9. A PSMZI device according to claim 1, further comprising: an even number of additional PDSs provided on either side of and point-symmetrically to the center PDS, each additional PDS comprising an upper waveguide arm and a lower waveguide aim; an even number of additional ACs or symmetric couplers (SCs) each comprising an upper waveguide portion and a lower waveguide portion; wherein one AC or SC is arranged directly on each side of each additional PDS, the upper and lower waveguide portions being respectively coupled to the upper and lower waveguide aims; and wherein the AC or SC on the one side of the additional PDS is point-symmetric to the AC or SC on the other side of the additional PDS.

10. A PSMZI device according to claim 1, wherein a width of each waveguide portion of each AC is between 1-3 μm.

11. A PSMZI device according to claim 1, wherein a width variation of each waveguide portion of each AC is between 10-1000 nm.

12. A PSMZI device according to claim 1, wherein a distance between the waveguide portions of each AC is between 0.25-0.5 μm.

13. A PSMZI device according to claim 1, further comprising: two coupler Mach Zehnder Interferometers (MZIs) arranged in a point-symmetric way, and wherein coupling coefficients C(λ) of each coupler MZI satisfy C=0.5 at a peak transmission wavelength of a cross-port of the coupler MZI, C=0 or C=1 at a peak transmission wavelength of a through-port of the coupler MZI, and dC/dλ=0 at the peak transmission wavelength of the cross-port.

14. A wavelength duplexer device comprising: at least one PSMZI device according to claim 1; and the wavelength duplexer device is configured for use in a passive optical network (PON) related application.

15. A method of fabricating a point-symmetric Mach-Zehnder Interferometer (PSMZI) device, the method comprising: providing three consecutive path delay sections (PDSs) as two outer PDSs and one center PDS, each PDS comprising an upper waveguide arm and a lower waveguide arm; providing four asymmetric couplers (ACs) each comprising an upper waveguide portion and a lower waveguide portion; wherein one AC is arranged directly on each side of each PDS, the upper and lower waveguide portions being respectively coupled to the upper and lower waveguide arms; wherein the AC on the one side of the PDS is point-symmetric to the AC on the other side of the PDS; and wherein the two ACs and the one outer PDS arranged on the one side of the center PDS are together point-symmetric to the two ACs and the one outer PDS arranged on the other side of the center PDS.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

[0055] FIG. 1 shows a PSMZI device according to an embodiment of the present invention;

[0056] FIG. 2 shows a PSMZI device according to an embodiment of the present invention;

[0057] FIG. 3 shows a PSMZI device according to an embodiment of the present invention;

[0058] FIG. 4 shows coupling in a coupler MZI device according to an embodiment of the present invention;

[0059] FIG. 5 shows an AC of the LSST type, as used in a PSMZI device according to an embodiment of the present invention;

[0060] FIG. 6(a) and FIG. 6(b) show simulation results for a PSMZI device according to an embodiment of the present invention;

[0061] FIG. 7(a) and FIG. 7(b) and FIG. 7(c) and FIG. 7(d) show simulation results for a PSMZI device according to an embodiment of the present invention;

[0062] FIG. 8(a) and FIG. 8(b) show simulation results for a PSMZI device according to an embodiment of the present invention;

[0063] FIG. 9(a) and FIG. 9(b) and FIG. 9(c) and FIG. 9(d) show simulation results for a PSMZI device according to an embodiment of the present invention; and

[0064] FIG. 10 shows a flow-diagram of a fabrication method according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0065] FIG. 1 illustrates schematically a PSMZI device 100 according to an embodiment of the present invention. As can be seen, the PSMZI device 100 comprises at least three consecutive PDSs 101, 102. In particular, the three PDSs 101, 102 are provided—in an extension direction of the PSMZI device 100 (i.e., from left to right in FIG. 1)—as a first outer PDS 102, a center PDS 101, and a second outer PDS 102 located on the other side of the center PDS 101 than the first outer PDS 102. Each PDS 101, 102 includes an upper waveguide arm 103 and a lower waveguide aim 104. The waveguide arms 103, 104 are more particularly made of a material having a refractive index in a range of 1.4-4.5. Advantageously, the waveguide arms 103, 104 may be made in a high refractive index contrast platform, more particularly are made of SiN.sub.x, and are optionally embedded into a cladding made of SiO.sub.2.

[0066] The PSMZI device 100 further comprises at least four ACs 105 (schematically illustrated in FIG. 1), provided alternatingly with the PDSs 101, 102. In particular, one AC 105 is arranged directly on each side of each PDS 101, 102. That means in the extension direction of the PSMZI device 100 (i.e., from left to right in FIG. 1)—at least a first AC 105, the first outer PDS 102, a second AC 105, the center PDS 103, a third AC 104, the second outer PDS 102, and a fourth AC 105 are consecutively arranged.

[0067] Each AC 105 includes an upper waveguide portion 106 and a lower waveguide portion 107 (only schematically illustrated in FIG. 1), and the upper and lower waveguide portions 106, 107 of an AC 105 are respectively coupled to the upper and lower waveguide arms 103, 104 of each neighboring PDS 101, 102. The four ACs 105 may advantageously be all of the LSST type.

[0068] In the PSMZI device 100, the AC 105 on the one side of each PDS 101, 102 is point-symmetric to the AC 105 on the other side of the PDS 101, 102. That means particularly, that the shape and asymmetry of the two ACs 105 around each PDS 101, 102 are inverted with respect to each other. Furthermore, the two ACs 105 and the first outer PDS 101 arranged on the one side of the center PDS 102 are together point-symmetric to the two ACs 105 and the second outer PDS 101 arranged on the other side of the center PDS 102. That means, for instance, that a path difference provided by the first outer PDS 102 is the same, but is provided in a different waveguide arm 103 than a path difference provided by the second outer PDS 102 (which is provided in the other waveguide arm 104).

[0069] FIG. 2 shows a more detailed embodiment of the PSMZI device 100 of FIG. 1. It can be seen that the PSMZI device 100 includes at least one IN port 304, IN_X port 305, THROUGH port 302 and CROSS port 303. Further, the PSMZI device 100 includes two coupler MZIs 301, which arranged in a point-symmetric way in respect to the central PDS 102. The first coupler MZI 301 includes the first outer PDS 102 and two of the ACs 105. The second coupler MZI 301 includes the second outer PDS 102 and the other two of the ACSs 105.

[0070] The structure of the PSMZI device 100 may be advantageously globally point-symmetric with the following characteristics: [0071] The path length from the IN port 304 to the THROUGH port 302 may optionally be the same as the total path length from the IN_X port 305 to the CROSS port 303. [0072] The path length difference ΔL in the center PDS 101 section is optionally zero (ΔL=0). That is, the center PDS 101 provides a path difference ΔL of zero. [0073] Optionally, a path difference ΔLc provided by the first outer PDS 102 is the same, but is provided in the other waveguide atm 103, 104, than a path difference provided by the second outer PDS 101. [0074] A total path length of all upper waveguide arms 103 may optionally be the same as a total path length of all lower waveguide arms 104. [0075] Advantageously there may be the same number of couplers on either side of the center PDS 101. In other words, the total structure may optionally have an even number, and specifically at least four, ACs 105.

[0076] Accordingly, also the coupler MZIs 301 on either side of the center PDS 102 are arranged in a point-symmetric layout.

[0077] Furthermore, the single MZIs 301 on either side of the central PDS 101 can be replaced with multiple-stage cascaded MZIs 301 (not shown). That means, the PSMZI device 100 may further comprise an even number of additional PDS on either side of and point-symmetrically to the center PDS 101, and an even number of additional ACs or SCs arranged directly on each side of each additional PDS. Thereby, the AC or SC on the one side of each additional PDS may be point-symmetric to the AC or SC on the other side of the additional PDS. Further, as shown in FIG. 4, each additional AC or SC includes—as each AC 105 of FIG. 2—an upper waveguide portion and a lower waveguide portion, and each additional PDS includes—as each PDS 101, 102 of FIG. 2—an upper waveguide arm and a lower waveguide arm. The upper and lower waveguide portions of the (directional) couplers are respectively coupled to the upper and lower waveguide arms of the PDS.

[0078] As shown in FIGS. 2 and 3, the individual ACs 105 (labelled as C1-C4 in FIG. 3) have a coupling coefficient K(λ), and are arranged in a fully point-symmetric way. The ACs 105 may advantageously be designed to have a coupling coefficient K(λ) that comes as close as possible to the optimal design (i.e. the optimal coupling coefficient). Thereby, the improved designing is supported by the additional design flexibility granted by the ACs 105.

[0079] FIG. 4 shows in the upper part schematically a coupler MZI 301 with a CROSS port 401 and a THROUGH port 402. In particular, FIG. 4 shows, how the shape and geometry of the ACs 105 are reversed on either side of a PDS 101, 102 in the coupler MZI 301. Due to this reversal, a phase deviation caused by each single AC 105 is compensated by another AC 105. Therefore, in summary no phase deviation is introduced, while it is possible to benefit fully from the design flexibility that the ACs 105 provide.

[0080] FIG. 4 shows, how the coupling between ACs 105 and PDS 101, 102, respectively (which is only shown schematically in FIG. 1), is implemented in detail. In particular, coupling between two ACs 105 and one of the outer PDS 101 is shown in FIG. 4. It can be seen that the upper waveguide portions 106 of the ACs 105 are connected to opposite sides of the upper waveguide aim 103 of the PDS 101. Likewise, the lower waveguide portions 107 of the ACs 105 are connected to opposite sides of the lower waveguide aim 104 of the PDS 101. The coupling is implemented identically for all the ACs 105 and PDSs 101, 102, respectively, of the PSMZI device 100 that is shown in FIG. 1.

[0081] Optionally, as also shown in FIG. 4, each coupler MZI 301 has the same coupling coefficient C(λ) at the CROSS port 401, which is given by:

C=4K(1−K)cos.sup.2(πn.sub.effΔLc/λ)

[0082] Further, the whole PSMZI device 100 has a coupling coefficient T(λ) at the CROSS port 303, which is given by:

T=4C(1−C)

[0083] For an optimal design, particularly for a broadband, flat-top, low-cross-talk spectral response of T(λ), the coupling coefficient C(λ) of each coupler MZI 301 may advantageously satisfy C=0.5 at a peak transmission wavelength of the CROSS port 401, C=0 or C=1 at a peak transmission wavelength of a THROUGH port 402, and dC/dλ=0 at the peak transmission wavelength of the CROSS port 401.

[0084] In order to fully exploit the increased design flexibility, which the use of ACs 105 in the PSMZI device 100 provides, the structural parameters ‘coupler waveguide width (w)’, ‘waveguide width difference (δw)’, and ‘gap width (δx)’ are advantageously selected. An AC 105 of the LSST type is shown as an example in FIG. 5, and the above-mentioned parameters w, δw and δx are indicated. That is, w is the basic width of the waveguide portions 106 and 107 (without taking into account asymmetries). Further, δx is the distance between the waveguide portions 106 and 107. Finally, δw is the difference caused by the asymmetries in comparison with the basic width w in each waveguide portion 106, 107. These three parameters can particularly be optimized to achieve a coupling coefficient K(λ) of the AC 105 that is as close as possible to the optimal coupling coefficient K.

[0085] FIG. 6(a) and FIG. 6(b) show the simulated coupling coefficient K over the TX wavelength band of (a) a single DC, and (b) a single AC 105 used in a PSMZI device 100 for a GPON duplexer application.

[0086] FIG. 6 also shows the fabrication tolerances of the PMSZI device 100 regarding waveguide width (dw) and height (dH). The solid curves correspond to the target design. The dotted lines correspond to a SC with a waveguide width error of dw=−20 nm and a waveguide height error of dH=10 nm, and respectively with dw=20 nm and dH=−10 nm. The dashed lines correspond to dw=−20 nm and dH=−10 nm, and respectively to dw=20 nm and dH=10 nm.

[0087] Ideally, K should be equal to 0 or 1 at a wavelength of 1.49 μm (i.e. the GPON TX band central wavelength). The comparison between the FIGS. 6 (a) and (b) demonstrates that the ACs 105 can be designed to have a coupling coefficient K with a value much closer to the ideal design compared to the SC, with at the same time a lower sensitivity to the waveguide dimension errors (dw, dH) at TX band.

[0088] FIG. 7(a) and FIG. 7(b) and FIG. 7(c) and FIG. 7(d) show simulated CROSS/THROUGH port transmission spectra of a PSMZI device using SCs and of a PSMZI device 100 using ACs 105 at the GPON TX wavelength band. Specifically, FIG. 7 (a) relates to the THROUGH port (TX insertion loss) with SCs. FIG. 7 (b) relates to the CROSS port (TX to RX cross talk) with SCs. FIG. 7 (c) relates to the THROUGH port 302 (TX insertion loss) with ACs 105. FIG. 7 (d) relates to the CROSS port 303 (TX to RX cross talk) with ACs 105.

[0089] FIG. 7(a) and FIG. 7(b) and FIG. 7(c) and FIG. 7(d) show again fabrication tolerances in respect to waveguide width (dw) and height (dH). The solid curves correspond to the target design. The dotted lines correspond to couplers with a waveguide width error of dw=−20 nm and a waveguide height error of dH=10 nm, and respectively with dw=20 nm and dH=−10 nm. The dashed lines correspond to couplers with dw=−20 nm and dH=−10 nm, and respectively with dw=20 nm and dH=10 nm.

[0090] The comparison between FIGS. 7 (a) and (c) shows that a PSMZI device 100 using ACs 105 can be designed to have lower insertion loss with a better fabrication tolerance at the TX band compared to a PSMZI device with SCs. The comparison between FIGS. 7 (b) and (d) shows that a PSMZI device 100 using ACs 105 can be designed to have a lower cross-talk at the TX band compared to a PSMZI device using SCs.

[0091] The technique to shape the coupling coefficient K of an AC 105 is specifically as follows. Firstly, on a standard platform, the structural parameters (e.g. coupler waveguide width, waveguide length, gap width) of a SC, with a coupling coefficient K being reasonably close to the optimal value, are obtained. Secondly, using this SC design as a starting point, the three crucial structural parameters (i.e. coupler waveguide width w, waveguide width difference δw, gap width δx, as show in FIG. 5) of an AC 105, such as for example an LSST type AC, are obtained. This may advantageously be done by parametrical optimization, and in order to bring K as close as possible to the optimal value (i.e. the optimal coupling coefficient).

[0092] The present invention can also be applied to 10GPON applications. In this respect, FIG. 8(a) and FIG. 8(b) show simulated coupling coefficients K over the TX wavelength band of (a) a single SC, and (b) a single AC 105 as used in a PSMZI device 100 for a 10GPON duplexer application.

[0093] FIG. 8(a) and FIG. 8(b) also shows fabrication tolerances in respect to waveguide width (dw) and height (dH). The solid curves are the target design. The dotted lines correspond to a coupler with a waveguide width error of dw=−20 nm and a waveguide height error of dH=10 nm, and respectively with dw=20 nm and dH=−10 nm. The dashed lines correspond to dw=−20 nm and dH=−10 nm, and respectively to dw=20 nm and dH=10 nm.

[0094] Ideally, K should be equal to 0 or 1 at a wavelength of 1.578 μm (i.e. the 10GPON TX band central wavelength). The comparison between FIGS. 12 (a) and (b) shows that the ACs 105 can be designed to have a coupling coefficient K with a value closer to the ideal design than the SCs, while at the same time having a lower sensitivity to the waveguide dimension errors (dw, dH) at the TX band.

[0095] FIG. 9(a) and FIG. 9(b) and FIG. 9(c) and FIG. 9(d) show simulated CROSS/THROUGH port transmission spectra of a PSMZI device using SCs and of a PSMZI device 100 using ACs 105 at the 10GPON TX wavelength band. FIG. 9 (a) relates to the THROUGH port (TX insertion loss) with SCs. FIG. 9 (b) relates to the CROSS port (TX to RX cross talk) with SCs. FIG. 9 (c) relates to the THROUGH port 302 (TX insertion loss) with ACs 105. FIG. 9 (d) relates to the CROSS port 303 (TX to RX cross talk) with ACs 105.

[0096] FIG. 9(a) and FIG. 9(b) and FIG. 9(c) and FIG. 9(d) also shows the fabrication tolerances in respect to waveguide width (dw) and height (dH). The solid curves correspond to the target design. The dotted lines correspond to couplers with a waveguide width error of dw=−20 nm and a waveguide height error of dH=10 nm, and respectively with dw=20 nm and dH=−10 nm. The dashed lines correspond to dw=−20 nm and dH=−10 nm, and respectively to dw=20 nm and dH=10 nm.

[0097] The comparison between FIGS. 9 (a) and (c) shows that a PSMZI device 100 using ACs 105 can be designed to have a lower insertion loss, and a better fabrication tolerance, at the TX band compared to a PMSZI device using SCs. The comparison between FIGS. 9 (b) and (d) shows that a PSMZI device 100 using ACs 105 can be designed to have lower cross-talk at the TX band compared to a PSMZI device using SCs.

[0098] FIG. 10 shows a method 500 of fabricating a PSMZI device 100 described above. In particular, in a step 501, the three consecutive PDSs 101, 102 are provided as the two outer PDS 102 and the one center PDS 101. Each PDS 101, 102 includes an upper waveguide arm 103 and a lower waveguide arm 104. In a further step 502, the four ACs 105 are provided, each AC 105 including an upper waveguide portion 106 and a lower waveguide portion 107.

[0099] In particular, the steps 501 and 502 include a step 503, in which one AC 105 is arranged directly on each side of each PDS 101, 102, the upper and lower waveguide portions 106, 107 being respectively coupled to the upper and lower waveguide arms 103, 104. Thereby, a step 504 ensures that the AC 105 on the one side of each PDS 101, 102 is point-symmetric to the AC 105 on the other side of the PDS. Another step 505 ensures that the two ACs 105 and the one outer PDS 102 arranged on the one side of the center PDS 101 are together point-symmetric to the two ACs 105 and the one outer PDS 102 arranged on the other side of the center PDS 101.

[0100] In the method 500, the ACs 105 and PDS 101, 102 can be fabricated before arranging them all in the point-symmetric and consecutive order, or can be designed one after another in the consecutive order, or can be arranged in the consecutive order and finally shaped to become point symmetric.

[0101] In summary, a PSMZI device 100 according to an embodiment of the present invention, i.e. particularly the use of ACs 105 in this PSMZI device 100, results in much lower insertion loss in a TX (transmitter) wavelength band, and in much lower TX to RX (receiver) cross-talk in a TX wavelength band. This also means that the PSMZI device 100 can be fabricated without an anti-reflection coating (ARC) step. As a consequence, production costs are saved and the process flow is simplified. Additionally, the PSMZI device 100 is much less sensitive to fabrication errors, and offers a larger flexibility in its design.

[0102] The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.